This invention relates to a floppy disk drive unit which can read and write information in different recording periods from and on floppy disks of a magnetic media for which recording periods are different such as for 1 megabyte and 2 megabytes, and more particularly to a circuit therefor which is improved in its reading performance.
A floppy disk drive unit as a general magnetic recording and reproducing apparatus has come into wide use as a means for recording and reproducing information for use with a personal computer, a word processor, and so on. As general information is diversified and increased in density, a magnetic disk (floppy disk) as storing means for information is required to be reduced in size and have an increased capacity for recording information.
Such a requirement is partially met as the technical development of magnetic recording and reproducing apparatus has progressed of late: for example, a magnetic disk of a size of 3.5 inches can store information of 2 megabytes.
However, the recording period of a floppy disk drive unit for recording and reproducing information of such a lage capacity as 2 megabytes is about one half to that of a floppy disk drive unit for such a small capacity as 1 megabyte, and magnetic heads fo such recording periods are different in recording and reproducing frequency characteristics. Accordingly, information recorded in a longer recording period on a magnetic disk cannot be reproduced nor re-recorded on a floppy disk drive unit for a shorter recording period, and hence it must be rewritten once in a fomt of the shorter recording period, resulting in inconvenience.
Accordingly, a floppy disk drive unit is required to allow reading and writing with a single system of a magnetic disk which is recorded in a different recording period.
Now, a conventional floppy disk drive unit will be described with reference to FIG. 4 which illustrates a construction of a circuit for reading and writing information for 2 megabytes.
Referring to Fi. 4, the circuit includes a magnetic head 1, a change-over circuit 2, a preamplifier 3, a reading circuit 5, a writing circuit 8 and a filter circuit 9.
The magnetic head 1 writes andreads magnetic record data on and from a magentic disk. The change-over circuit 2 operates in response to a changing over instruction from a host computer not shown to change over to couple singals read by the magnetic head 1 to the preamplifier 3 or alternatively to couple signals fro the writing circuit 8 to the magnetic head 1.
Signals inputted from the magnetic head 1 to the preamplifier 3 via the change-over circuit 2 are amplified to a desired voltage and outputted to the filter circuit 9.
The filter circuit 9 receives signals from the preamplifier 3 over two signal lines. To one signal line, a capacitor C1, a resistor R1 and an inducer L1 are connected to series while a capacitor C2, a resistor R2 and an inductor L2 are connected in series to the other signal line. A capacitor 7 is connected in parallel with the two signl lines between junctions between the resistor R1 and the inductor L1 and between the resistor R2 and the inductor L2. A capacitor C6 and a resistor R9 are connected in parallel relationship between one ends of the inductors L1 and L2, that is, at junctions between the filter circuit 9 and the reading circuit 5. Thus, DC voltage components and very low freuency components of signals from the preamplifier 3 are cut by the capacitors C1 and C2 while high frequency components are cut by the inductors L1 and L2 and the capacitors C6 and C7 which interconnect corresponding ends of th inductors L1 and L2. General voltage divisionis attained by the resistors R1 and R2 and the resistor R9 to produce a predetermined output level. The filter circuit 9 having such a construction as described above filters only a frequency band necessary for reading the data and outputs it to the reading circuit 5.
The reading circuit 5 differentiates signals from the filer circuit 9 and shapes waveforms of the signals into pulse signals corresponding to digital codes of 0 to 1. The pulse signals are outputted to the host computer (not shown).
The writing circuit 8 converts input data from the host computer into a level of voltage in accordance with recording andreproducing characteristics of a magnetic disk for 2 megabytes and of the magnetic head 1. The input data thus converted is delivered to the magnetic head 1 via the change-over circuit 2. In this instance, the change-over circuit 2 has been changed over, in response to an instruction from the host computer, to connect the magnetic head 1 to the writing circuit 8.
However, a magnetic recording and reproducing apparatus adapted for a short recording period cannot reproduce from a magnetic disk which is recorded in a long recording period.
This problem will now be described with reference to FIGS. 4 and 5.
FIG. 5 is a diagram illustrating operations of several parts when a magentic disk which is recorded for 1 megabytes is reproduced on a different floppy disk drie unit for 2 megabytes.
On a magentic disk, information is recorded in the form of reversals and non-reversals of polarity of magnetization as shown by "N" and "S" in (c) of FIG. 5 which correspond to digitial codes indicated by "1" and "0" in (a) of FIG. 5. The information recorded on the magnetic disk is detected by a magentic head 1 and is outputted in the form of signal as shown in (e) of FIG. 5 to a reading circuit 5 by way of a filter circuit 9. A portion of the signal (e) called a shoulder as indicated by S1 to S5 is caused due to an excessively high resolution of reading characteristics of he magnetic head for 2 megabytes relative to the writing characteristics of the magnetic head for 1 megabyte. The signal (e) is differentiated by the reading circuit 5 into a signal waveform as shown in (f) of FIG. 5. The signal waveform (f) thus differentiated is then converted into a square wave signal as shown in (g) of FIG. 5 by a zero volt comparator or the like of the reading circuit 5 and is then pulse shaped into pulse signals (h) as shown in (h) of FIG. 4 in accordance with variations in level of the square wave signal (g). The pulse signals (h) involve error pulses E1 to E5 corresponding to the shoulders S1 to S5 as described above. If the error pulses E1 to E5 are inputted to the host computer, this will result in inputting of unnecessary additional pulses for a period for 1 megabyte to the host computer. As a result, an output of the host computer will involve error portions as shown by E in (i) of FIG. 5, and the error portions will be outputted as 1 despite that the magnetization polarity as shown in (c) of FIG. 5 is not reversed as in the error pulses E2 and E4, causing an error in operation.
When the frequency characteristics are not in confomity with each other in this manner, conventionally a method of lowering the high frequency band of the filter characteristics is employed. Thus, in an apparatus which is required to be able to reproduce in different recording periods such as 1 megabyte and 2 megabytes with a single apparatus, it seems advisable to change over the filter by means of a switching element to select a frequency band necessary for the required reproduction. Such an apparatus, however, has various problems as desdribed below.
At first, the apparatus necessitates a number of filter circuits corresponding to that of different recording periods which circuits have a filter characteristic conforming to the detection sensitivity ]of the reading circuit. Further, where a switching element for changing over among the filter circuits is constituted by a bipolar transistor, a base current for interrupting between the emitter and collector will flow into a fignal line, resulting in reduction of Q of the filter circuits. This problem can be resolved if a coupling capacitor of a large capacity is used to separate the base current from the signal line, but anyway, a dispersion will be caused to appear in the central value of signal levels due to a difference in base current between a pair of transistors corresponding to a pair of signal lines, and besides an additional resistor for base biasing or some other additional elements are required. Thus, this resolution is not practical.
Meanwhile, where a switching element is constituted from a mechanical relay or the like, such a problem arising from a base current can be eliminated, but this arrangement also presents various problems that it requires an additional space for installation, that it involves mechanical deteriorations, that noises are induced from the switching element, and so on. Accordingly, this arrangement is less practical.